1. Field of the Invention
The present invention relates to dielectric ceramic compositions which are suitably used in a high frequency band, such as a microwave or a milliwave band, and which are used for forming microwave resonators, filters and laminated capacitors, and relates to a dielectric ceramic compact and multilayer ceramic substrates, ceramic electronic devices, and laminated ceramic electronic devices formed of the dielectric ceramic compacts.
2. Description of the Related Art
In order to miniaturize electronic devices, such as a microwave resonator and a filter, a structure formed of a dielectric ceramic material having a high relative dielectric constant has been proposed in place of a cavity resonator. When the relative dielectric constant of a dielectric material is represented by e, by exploiting the effect where the wavelength of an electromagnetic wave in the dielectric material is shortened by a factor of (1/∈)xc2xd of the wavelength in free space, miniaturization of a microwave resonator and a filter has been attempted by using a dielectric ceramic material having a high relative dielectric constant.
However, the relative dielectric constant ∈ of a dielectric ceramic having a temperature coefficient which can be practically used for a dielectric resonator has been up to 100 or less, and as a result, it has been difficult to meet the requirement of further miniaturization.
Accordingly, a method using an LC resonator which is a known microwave circuit has been proposed to perform further miniaturization under the limitations of the relative dielectric constant of the dielectric ceramic. That is, by applying a laminating method which is practically used for forming laminated capacitors and multilayer substrates to the formation of an LC circuit, microwave electronic devices can be even further miniaturized and the reliability thereof can be improved.
However, in order to obtain an LC resonator having a high Q in a microwave band, the electrical conductivity of internal electrodes embedded in a laminated capacitor or in a multilayer circuit substrate must be high. As an internal electrode which can be simultaneously fired together with a dielectric material or a multilayer circuit substrate, a metal having a high electrical conductivity, such as gold (Au), silver (Ag) or copper (Cu), must be used.
Accordingly, the dielectric ceramic compact must have a high relative dielectric constant, a high Q, and a small temperature coefficient thereof, and in addition to these, the dielectric ceramic compact must be a material which can be obtained by co-sintering together with an internal electrode composed of a metal having a low melting point. A material which can meet all of these requirements has not been obtained up to now.
For example, since a metal such as Ag, Au or Cu has a melting point of approximately 960 to 1,063xc2x0 C., and a conventional dielectric ceramic composition has a high firing temperature of 1,350xc2x0 C. or more, co-sintering cannot be performed with the metal having superior electrical conductivity.
Accordingly, an object of the present invention is to provide a dielectric ceramic composition which has a high relative dielectric constant, a high Q, and a small temperature coefficient after sintering and, in addition, which can be sintered at a relatively low temperature.
Another object of the present invention is to provide a dielectric ceramic compact obtained by sintering the dielectric ceramic composition, a multilayer ceramic substrate, a ceramic electronic device and a laminated ceramic electronic device, which are formed of the dielectric ceramic compact described above and which has superior high frequency properties.
To these ends, in accordance with one aspect of the present invention, a dielectric ceramic composition comprises a BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition represented by the formula xBaOxe2x80x94yTiO2xe2x80x94zReO3/2 and a glass composition; wherein, in the formula xBaO-yTiO2xe2x80x94zReO3/2, 8xe2x89xa6xxe2x89xa618, 52.5xe2x89xa6yxe2x89xa665 and 20xe2x89xa6zxe2x89xa640, x, y, and z being mole percent, x+y+z=100, and Re indicates a rare earth element, and the glass composition comprises about 10 to 25 wt % of SiO2, about 10 to 40 wt % of B2O3, about 25 to 55 wt % of MgO, 0 to about 20 wt % of ZnO, 0 to about 15 wt % of Al2O3, about 0.5 to 10 wt % of Li2O and 0 to about 10 wt % of RO in which R is at least one selected from the group consisting of Ba, Sr and Ca.
In the dielectric ceramic composition described above, the glass component is preferably Pb-free glass. In addition, the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based dielectric ceramic is preferably Bi-free dielectric ceramic.
In addition to the primary component composed of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based dielectric ceramic and the glass component, the dielectric ceramic compact described above may further comprise CuO as a subcomponent.
In addition to the primary components composed of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based dielectric ceramic and the glass component, the dielectric ceramic composition described above may further comprise TiO2 as a subcomponent.
In the dielectric ceramic composition described above, the content of the glass composition is preferably in the range of from about 15 to 35 wt % with respect to about 65 to 85 wt % of the BaOxe2x80x94TiO2xe2x80x94ReO3-based ceramic composition.
In accordance with another aspect of the present invention, a dielectric ceramic composition comprises a BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition represented by the formula xBaOxe2x80x94yTiO2xe2x80x94ReO3/2, a glass composition, CuO, and TiO2; wherein 8xe2x89xa6xxe2x89xa618, 52.5xe2x89xa6yxe2x89xa665, and 20xe2x89xa6zxe2x89xa640, x, y, and z being mole percent and x+y+z=100, and Re indicates a rare earth element, and the glass composition comprises about 10 to 25 wt % of SiO2, about 10 to 40 wt % of B2O3, about 25 to 55 wt % of MgO, 0 to about 20 wt % of ZnO, 0 to about 15 wt % of Al2O3, about 0.5 to 10 wt % of Li2O, and 0 to about 10 wt % of RO in which R is at least one selected from the group consisting of Ba, Sr and Ca, and the contents of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition, the glass composition, the TiO2 and the CuO are about 65 to 85 wt %, about 15 to 35 wt %, about 0.1 to 10 wt %, and about 3 wt % or less, respectively.
In the dielectric ceramic composition described above, the glass component is preferably Pb-free glass. In addition, the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based dielectric ceramic is preferably Bi-free dielectric ceramic.
Since the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition represented by the specific formula xBaOxe2x80x94yTiO2xe2x80x94zReO3/2 and the specific glass composition described above are used as primary materials, as will be apparent in examples described later, sintering at a low temperature of not more than 1100xc2x0 C., preferably 1000xc2x0 C. or less, can be performed, and hence, co-sintering with a metal having superior conductivity, such as Ag, Au or Cu, can also be performed.
In addition, a dielectric ceramic compact, which obtained by sintering the dielectric ceramic composition, can be obtained having a small temperature coefficient and a high relative dielectric constant in a high frequency band, more specifically, in a microwave band and a milliwave band.
Furthermore, when the glass component composed of the glass composition is crystallized, or the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition and the glass composition form a crystal phase by reaction with each other, a crystal phase having a high Q, such as Mg2B2O5, Mg3B2O6, BaTi4O9, Ba2Ti9O20, Mg2TiO4,Mg2SiO4, Zn2TiO4, Zn2Ti3O8 or ZnAl2O4 is precipitated, whereby a dielectric ceramic compact having a high Q can be obtained.
The rare earth element Re used for the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition is not specifically limited, and for example, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu may be optionally used alone or in combination.
The reasons the composition represented by xBaOxe2x80x94yTiO2xe2x80x94zReO3/2 are used for the dielectric ceramic compact of the present invention are described below.
FIG. 1 is a ternary composition diagram of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition. In this ternary composition diagram, the area surrounded by the solid line P corresponds to the composition represented by the xBaOxe2x80x94yTiO2xe2x80x94zReO3/2.
For a composition in area A shown in FIG. 1, that is, for a composition in the area in which x is 18 or more, it is difficult to form the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition by sintering, and a porous ceramic can only be obtained even at a firing temperature of 1,400xc2x0 C. In area B in which y is more than 65 and z is less than 20, the temperature properties are inferior. When a multilayer circuit substrate containing a capacitor therein is formed, the temperature coefficient of capacitance (TCC) is excessively negative. In area C in which x is less than 8, the relative dielectric constant of the obtained ceramic compact is excessively decreased and sintering properties thereof are also unstable. In addition, in area D in which z is more than 40 and y is less than 52.5, the temperature coefficient of capacitance is excessively positive and the relative dielectric constant is also decreased.
In the present invention, the specific BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition and the specific glass composition described above constitute the primary component as described above.
The glass composition comprises about 10 to 25 wt % of SiO2, about 10 to 40 wt % of B2O3, about 25 to 55 wt % of MgO, 0 to about 20 wt % of ZnO, 0 to about 15 wt % of Al2O3, about 0.5 to 10 wt % of Li2O and 0 to about 10 wt % of RO in which R is at least one selected from the group consisting of Ba, Sr and Ca. In this connection, ZnO and Al2O3 are additive components which may be or may not be present.
The B2O3 serves to decrease the glass viscosity and to facilitate sintering of the ceramic composition and the glass composition. In addition, B2O3 forms a crystal having a high Q, such as Mg2B2O5 or Mg3B2O6. However, when the content of B2O3 is more than about 40 wt %, the humidity resistance is degraded and when the content is less than about 10 wt %, sintering cannot be performed below 1100xc2x0 C.
The SiO2 forms a crystal having a high Q, such as Mg2SiO4. However, when the content of SiO2 is more than about 25 wt %, since a softening temperature of the glass is excessively increased, the sintering properties of the ceramic composition and the glass composition are degraded, and when the content is less than about 10 wt %, the humidity resistance is degraded.
MgO serves to facilitate the reaction between the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition and the glass composition and serves to decrease the softening temperature of the glass composition. In addition, MgO forms a crystal having a high Q, such as Mg2B2O5, Mg3B2O6, Mg2TiO4 or Mg2SiO4. When the content of is less than about 25 wt %, the sintering properties are degraded, and hence, sintering cannot be performed below 1100xc2x0 C. In addition, when the content of MgO is more than about 55 wt %, the humidity resistance is degraded to some extent, and in addition, it is difficult to perform vitrification.
Li2O serves to decrease a softening temperature of the glass. When the content of Li2O is more than about 10 wt %, the humidity resistance is degraded to some extent, and when the content is less than about 0.5 wt %, the softening temperature is excessively increased so that sintering cannot be performed.
ZnO serves to increase the Q; however, when the content thereof is more than about 20 wt %, the sintering properties are degraded. In addition, Al2O3 serves to improve the humidity resistance; however, when the content is more than about 10 wt %, the sintering properties are degraded. ZnO forms a crystal having a high Q, such as Zn2TiO4, Zn2Ti3O8 or ZnAl2O4.
BaO, CaO and SrO serve to improve sintering properties; however, when the contents thereof are more than about 10 wt %, the Q is decreased. Particularly, BaO forms a crystal having a high Q, such as BaTi4O9 or Ba2Ti9O20.
The dielectric ceramic composition of the present invention preferably further comprises CuO as a subcomponent in addition to the primary component described above. The CuO as the subcomponent serves as an auxiliary sintering agent. However, when the content of CuO is more than about 3 wt %, the Q is decreased, and hence, the temperature coefficient of capacitance may be excessively positive in some cases.
The dielectric ceramic composition of the present invention may further comprise TiO2 as a subcomponent in addition to the primary component described above, and TiO2 serves to facilitate crystallization of glass. However, when the content of TiO2 is more than about 10 wt % of the dielectric ceramic compact, the sintering properties may be degraded in some cases.
In the dielectric ceramic compact of the present invention, when the content of the glass composition is less than about 15 wt % of the entire dielectric ceramic compact, it may be difficult to perform sintering in some cases and when the content is more than about 35 wt %, the humidity resistance may be degraded or the relative dielectric constant may be decreased in some cases. Accordingly, about 15 to 35 wt % of the glass composition is preferably contained with respect to about 65 to 85 wt % of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition.
Furthermore, the dielectric ceramic compact of the present invention preferably comprises about 65 to 85 wt % of the BaOxe2x80x94TiO2xe2x80x94ReO3/2-based ceramic composition, about 15 to 35 wt % of the glass composition, about 0.1 to 10 wt % of TiO2 and about 3 wt % or less of CuO.
In accordance with another aspect of the present invention, a multilayer ceramic substrate is provided having a dielectric ceramic layer comprising the dielectric ceramic compact of the present invention and electrodes provided on the ceramic substrate. In this multilayer ceramic substrate, since the dielectric ceramic layer comprise the dielectric ceramic compact of the present invention, and the electrodes are formed on the dielectric ceramic layer, the multilayer ceramic substrate can be obtained by firing at a low temperature, such as 1100xc2x0 C. or less, and as a result, a multilayer ceramic substrate having a high dielectric constant, a high Q and a small temperature coefficient of dielectric properties can be obtained.
The multilayer ceramic substrate described above may further comprise another dielectric ceramic layer having a dielectric constant lower than that of the dielectric ceramic layer of the invention.
The electrodes of the multilayer ceramic substrate described above may be disposed so as to oppose each other with at least a part of the dielectric ceramic layer provided therebetween for forming a capacitor.
In the multilayer ceramic substrate described above, the electrodes may comprise a plurality of internal electrodes for forming a capacitor and a plurality of coil conductors in electrical contact with each other for forming an inductor.
In accordance with another aspect of the present invention, a ceramic electronic device is provided which comprises the multilayer ceramic substrate described above and at least one electronic element which is mounted on the multilayer ceramic substrate and which forms a circuit together with the electrodes.
Preferably, the ceramic electronic device described above may further comprise a cap fixed on the multilayer ceramic substrate so as to surround the electronic element. More preferably, the ceramic electronic device comprises an electrical conductive cap as the cap.
In addition, the ceramic electronic device described above may further comprises a plurality of external electrodes formed only on the bottom surface of the multilayer ceramic substrate described above and a plurality of throughhole conductors in electrical contact with the external electrodes described above and in electrical contact with the electrodes or the electronic element described above.
In accordance with yet another aspect of the present invention, a laminated ceramic electronic device is provided comprising a sintered ceramic body comprising the dielectric ceramic compact of the present invention, a plurality of electrodes disposed in the sintered ceramic body and a plurality of external electrodes which are provided on outside surfaces of the sintered ceramic body and which are each in electrical contact with one of the plurality of electrodes. The plurality of electrodes may comprise internal electrodes laminated to each other with at least a part of the sintered ceramic body provided therebetween so as to from a laminated capacitor unit. Separately or in addition to the internal electrodes for forming the laminated capacitor unit, the plurality of electrodes may further comprise coil conductors in electrical contact with each other so as to form a laminated inductor unit.